The invention relates to optical data and information storage systems, especially systems using lasers for encoding and reading.
In particular, the invention relates to a system for encoding and reading samples of analog information on an optical disc, in a manner that achieves high density and high precision. One important feature of the invention is the ability to achieve optical information storage at sufficient density that a video picture can be stored and generated in one revolution of a compact optical disc, with a 2" inner diameter for recording. A second feature is the ability to sample at a bandwidth greater than 10 MHz to obtain a high resolution video picture.
Optical data storage systems involve encoding of information by burning pits or exposing photoresist into the surface of a data storage medium, which may be a rotatable disc. The pits can either burn through the medium or create holes in a developed photoresist, creating a light transmissive opening which is read from the other side of the medium, or simply make a reflective medium surface nonreflective at the location of the pit, in systems which read reflected light. The pits, usually generated by a laser beam, can carry information either by digital storage encoding or analog encoding. In digital encoding, at each reading location there is either a pit or no pit. In analog systems, information can be represented by the length of a pit, or the distance between pits, or both.
In many typical systems prior to the present invention, analog information was contained in the length of each pit, and also repeated by the length between pits, FM modulation. The object was to reduce noise, by making a double encoding and a double reading of each sample, then averaging the two in the reading mode in order to reproduce the information more precisely. Although such systems have been effective in reducing noise, they have been inefficient in the sense that additional space is required for the double recording of each bit of information on the disc. In video discs, for example, this has led to the need for a 12" disc, with accurate encoding only possible beginning at about 4" of diameter and extending outwardly. By convention, one revolution of the disc is used to generate a video picture, or frame, in the CAV mode (constant angular velocity), and with the prior encoding systems there was not sufficient storage space available in a revolution inside the 4" diameter line.
U.S. Pat. No. 4,175,270 issued to George Zenzefilis, discloses an information storage system which utilizes a series of magnetic reference signals encoded on the magnetic medium and dividing up the storage medium, and which involves encoding of information in such a way as to straddle a magnetic reference signal located on the inner portion of the disc. (See, e.g., FIG. 5A). However, this is in the context of magnetic storage, not optical storage. The magnetic storage domain for each bit of information takes up considerably more medium space than does optical storage, presenting different considerations. The patent discloses a photographically-encoded embodiment, but not one using laser encoding and reading, especially of reflective surfaces.
Moreover, Zenzefilis encodes his "clock track" of reference signals at an inner location on a video disc, near the center, and his system requires two pickup heads, one in fixed radial position for detecting the clock track signals, and one positioned outwardly and radially movable for picking up the video information. Still further, in Zenzefilis' system each segment or sample of information is associated with an element of the video display, i.e., a pixel containing chroma and luminance information, and all segments are arrayed in radial lines along the clock signals so that discrete segments from circumferential path to path corresponding to a particular element or pixel are located at generally the same angular position from the center of rotation.
Prior information storage and retrieval systems, both magnetic and optical, have not been capable of the combined density, precision and adaptability enabled by the present invention described below.